A typical cellular wireless network includes a number of base stations each radiating to provide coverage in which to serve user equipment devices (UEs) such as cell phones, tablet computers, tracking devices, embedded wireless modules, and other wirelessly equipped communication devices. In turn, each base station may be coupled with network infrastructure that provides connectivity with one or more transport networks, such as the public switched telephone network (PSTN) and/or the Internet for instance. With this arrangement, a UE within coverage of the network may engage in air interface communication with a base station and may thereby communicate via the base station with various remote network entities or with other UEs served by the base station.
Such a system may operate in accordance with a particular air interface protocol, examples of which include, without limitation, Orthogonal Frequency Division Multiple Access (OFDMA) (e.g., Long Term Evolution (LTE) and Wireless Interoperability for Microwave Access (WiMAX)), Code Division Multiple Access (CDMA) (e.g., 1×RTT and 1×EV-DO), Global System for Mobile Communications (GSM), IEEE 802.11 (WIFI), BLUETOOTH, and others.
In accordance with the air interface protocol, a base station may provide service on one or more carriers, each spanning particular radio-frequency on which communications can flow wirelessly between the base station and UEs. Such a carrier could be structured to provide a downlink for carrying communications from the base station to UEs and an uplink for carrying communications from UEs to the base station. For instance, the carrier could be frequency division duplex (FDD), with separate frequency ranges provided respectively for downlink and uplink communication, or time division duplex (TDD), with a single frequency range being time division multiplexed between downlink and uplink use.
Through modulation or other means on the carrier, the downlink and uplink could then be structured to define various channels for carrying communications between the base station and UEs. For instance, the downlink could be structured to define one or more traffic channels for carrying bearer data from the base station to UEs and one or more control channels for carrying control signaling from the base station to UEs. And the uplink could define one or more traffic channels for carrying bearer data from UEs to the base station and one or more control channels for carrying control signaling from UEs to the base station.
Further, data transmitted over the downlink or uplink channels could be modulated according to various modulation and coding schemes (MCSs) having various orders of modulation. Modulating the data with a higher-order MCS could provide for more efficient transmission of the data, thereby helping to increase the effective bandwidth and associated throughput of data over the downlink or uplink channels. For instance, modulating data using 64 Quadrature Amplitude Modulation (64QAM) could allow for a higher transmission rate of the data than modulating the data using a lower-order MCS, such as 16QAM. Likewise, modulating data using 16QAM could allow for a higher transmission rate of the data than modulating the data using an even lower-order MCS, such as Quadrature Phase Shift Keying (QPSK) modulation. Other examples of MCSs can be used to modulate the data as well.
In practice, when a UE enters into coverage of a base station on a particular carrier, the UE may attach or register with the base station on that carrier, and the base station may then serve the UE on that carrier. Further, under certain air interface protocols, a base station may be able to serve a UE concurrently on multiple carriers, to help increase the effective bandwidth and associated throughput available to the UE. For instance, if a UE is attached with a base station on a first carrier, the base station may then add a second carrier to its service of the UE so as to then provide the UE with “carrier aggregation” service on a combination of the first carrier and the second carrier. In that arrangement, the first carrier may be considered the UE's primary carrier or primary cell (PCell), and the second carrier may be considered the UE's secondary carrier or secondary cell (SCell). Depending on the carrier aggregation implementation, the SCell might be used principally for downlink communication (to increase the UE's downlink throughput) rather than for uplink communication, and the PCell may carry some or all control signaling related to the SCell (in addition to control signaling related to the PCell).